Rampal Pandey1, Durgesh Singh2, Neha Thakur1, Krishna K Raj2. 1. Department of Chemistry, National Institute of Technology Uttarakhand, Srinagar, Uttarakhand 246174, India. 2. Department of Chemistry, Dr. Harisingh Gour University, Sagar, Madhya Pradesh 470003, India.
Abstract
Three 1D coordination polymers (CPs) [M(pdca)(H2O)2] n (M = Zn, Cd, and Co; 1-3), and a 3D coordination framework {[(CH3)2NH2][CuK(2,3-pdca)(pa)(NO3)2]} n (4) (2,3-pdca = pyridine-2,3-dicarboxylate and pa = picolinic acid), have been synthesized adopting a solvothermal reaction strategy. The CPs have been thoroughly characterized using various spectral techniques, that is, elemental analyses, FT-IR, TGA, DSC, UV/vis, and luminescence. Structural information on 1-4 was obtained by PXRD and X-ray single-crystal analyses, whereas morphological insights were attained through FESEM, AFM, EDX, HRTEM, and BET surface area analyses. Roughness parameters were calculated from AFM analysis, whereas dimensions of small domains and interplanar spacing were defined with the aid of HRTEM. CPs 1-3 are 1D isostructural networks, whereas 4 is a 3D framework. Moreover, 1-4 display moderate luminescence at rt. In addition, 1-4 have been applied as economic and efficient porous catalysts for the Knoevenagel condensation reaction and C-H bond activation under mild conditions with good yields (95-98 and 97-99%), respectively. Notably, 1-3 can be reused up to seven cycles, whereas 4 can be reused up to five catalytic cycles with retained catalytic efficiency. Relative catalytic efficacy toward the Knoevenagel condensation reaction follows in the order 2 > 1 > 3 > 4, whereas 2 > 4 > 1 > 3 for C-H activation. The present result demonstrates synthetic, structural, optical, morphological, and catalytic aspects of 1-4.
Three 1D coordination nclass="Chemical">polymers (class="Chemical">pan class="Chemical">CPs) [M(pdca)(H2O)2] n (M = Zn, Cd, and Co; 1-3), and a 3D coordination framework {[(CH3)2NH2][CuK(2,3-pdca)(pa)(NO3)2]} n (4) (2,3-pdca = pyridine-2,3-dicarboxylate and pa = picolinic acid), have been synthesized adopting a solvothermal reaction strategy. The CPs have been thoroughly characterized using various spectral techniques, that is, elemental analyses, FT-IR, TGA, DSC, UV/vis, and luminescence. Structural information on 1-4 was obtained by PXRD and X-ray single-crystal analyses, whereas morphological insights were attained through FESEM, AFM, EDX, HRTEM, and BET surface area analyses. Roughness parameters were calculated from AFM analysis, whereas dimensions of small domains and interplanar spacing were defined with the aid of HRTEM. CPs 1-3 are 1D isostructural networks, whereas 4 is a 3D framework. Moreover, 1-4 display moderate luminescence at rt. In addition, 1-4 have been applied as economic and efficient porous catalysts for the Knoevenagel condensation reaction and C-H bond activation under mild conditions with good yields (95-98 and 97-99%), respectively. Notably, 1-3 can be reused up to seven cycles, whereas 4 can be reused up to five catalytic cycles with retained catalytic efficiency. Relative catalytic efficacy toward the Knoevenagel condensation reaction follows in the order 2 > 1 > 3 > 4, whereas 2 > 4 > 1 > 3 for C-H activation. The present result demonstrates synthetic, structural, optical, morphological, and catalytic aspects of 1-4.
Development
of porous heterogeneous molecular catalysts is a demand
of current industrial research. nclass="Chemical">Metal–organic frameworks (class="Chemical">pan class="Chemical">MOFs)
frequently recognized as porous coordination polymers (PCPs) are materials
with excellent crystallinity composed of metal ions/metal clusters
with organic linkers.[1−6] Switchable MOFs are a type of smart materials that undergo distinct
and reversible structural changes upon exposure to the external stimuli,
thus finding interesting technological application.[7] The geometry of a ligand governs crystal topology of MOFs
and tunes micro-to-nano crystal morphologies.[8] For instance, the ZIF-MOF family offers excellent chemical and thermal
stability and adjustable porous structures.[9] Over past couple of decades, PCPs have drawn tremendous attention
and established prodigious worldwide interest not only owing to versatile
designability, excellent tunable porosity, structural diversity, miscellaneous
topologies, and high surface areas but also due to their fascinating
applications in waste water treatment,[10−13] catalysis and gas storage/separation,[14−17] chemical sensing,[18−20] optoelectronic materials,[21−26] heterogeneous catalysis,[27−34] water oxidation,[35,36] energy storage and conversion,[37−40] luminescent materials,[41−44] and so forth. CPs having metal nodes and a variety
of bridging organic linkers comprise exposed active metal centers,
thereby providing a high degree of metal dispersion for unambiguous
catalytic applications.[45,46] In addition, most emerging
and advanced applications of crystalline PCP materials are heterogeneous
catalysis.[59−67] Because of their multicomponent nature, feasible functionalized
catalytic active site PCP materials have been proved as ideal platforms
for heterogeneous catalysis and they have been utilized in a wide
range of chemical reactions with promising catalytic performance.[56,82,83] CPs have demonstrated excellent
morphology-dependent heterogeneous catalytic performance catalytic
activities for a wide variety of organic reactions, viz., aldol condensation,[57,58] Henry reaction,[59−61] Michael addition, multicomponent reaction,[62] C–H activation,[47,63] Friedel–Crafts reactions,[64] Tandem
reactions,[65] and Knoevenagel condensation.[66−70] Moreover, the structural tunability of PCPs leads to the elegant
tailoring of a chemical environment at catalytic sites, thereby causing
chemo, regio-, stereo-, and/or enantio-selectivities.[71−73] In addition, the crystalline nature of PCPs provides opportunity
to well-disperse active sites at a molecular level, thereby favoring
to the mechanistic studies.[48−55]
In particular to the Knoevenagel condensation reaction, a
variety
of heterogeneous catalysts have been used, that is, zeolites,[74−78] clays,[79−83] organic-functionalized molecular sieves, silicate–organiccomposite materials,[84] and nclass="Chemical">PCPs,[30,85−89] which demonstrated vast significant advantages over homogeneous
catalysts. Remarkably, it is noteworthy to mention that class="Chemical">pan class="Chemical">PCP-based
heterogeneous catalysts are quite limited toward oxidative conversion
of benzaldehyde into benzoic anhydride (C–H activation) which
is a very advantageous reagent in organic synthesis,[90−93] for instance, silyl ester’s lactonization,[94−96] asymmetric
esterifcations,[97] and synthesis of peptides
and drugs.[98−100] Employing homogeneous catalysts, Knoevenagel
condensation reactions and oxidative conversion of benzaldehyde into
benzoic anhydrides involve various difficulties such as low catalyst
loading, poor recyclability, tedious work-up process, longtime consumption,
and catalyst contamination, whereas heterogeneous catalysts are easily
recoverable and reusable and minimize the undesired waste.[101−105]
Additionally, nclass="Chemical">pyridine-dicarboxylate and analogous linkers
have
been meticulously utilized in the construction of a variety of class="Chemical">pan class="Chemical">PCPs.[109−119,156] Among pyridine-dicarboxylates,
2,3-pdca ligand has been least utilized toward production of PCPs
possibly due to obstruction of its use as a bridging ligand.[118,156] Notably, under hydrothermal/solvothermal conditions (Scheme ),[106−108] 2,3-pdca rarely assumes a unique coordination mode via in situ mono-decarboxylation
at the second position, thereby producing nicotinic acid,[156] whereas 3,4-pdca transforms into isonicotinic
acid.[119] Remarkably, the present work entails
in situ decarboxylation at the third position on 2,3-pdca to produce
picolinic acid (pa) under the solvothermal reaction[120] (Chart c and Scheme ). Although
2,3-pdca-containing MOFs/CPs are documented in the literature, however
these have seldom been used as heterogeneous catalysts especially
in the Knoevenagel condensation reactions and oxidative conversion
of benzaldehyde into benzoic anhydrides.[63,121,122]
Scheme 1
In Situ Mono-decarboxylation
of 2,3-pdca to Form (Right) Nicotinic
Acid (∼Half Dozen Examples) and (Left) Picolinic Acid in the
Present Work
Chart 1
Binding Modes of 2,3-pdca in 1–3 (a) and 4 (b,c).
Scheme 2
Synthetic Route for
Preparation of 1–4
Results
and Discussion
FT-IR Spectral Analyses
The FT-IR
spectrum of 1 displayed broad and strong vibration at
3418 cm–1 which may be ascribed to the existence
of coordinated nclass="Chemical">water molecules (Figure S19.1).[124] Furthermore, the presence of the
strong peaks at 1667 and 1587 cm–1 may be associated
with the characteristic of asymmetric stretching (νas) vibrations due to the −COO– group, whereas
the peaks observed at 1442 and 1394 cm–1 are attributed
to symmetric stretching (νs) vibration of the −COO– group. Relatively larger seclass="Chemical">parations >200 cm–1 class="Chemical">pan class="Gene">between νas(COO–) and νs(COO–) indicate the monodentate
coordination
mode of the carboxylate group, whereas smaller separation <200
cm–1 suggests the chelating bidentate coordination
mode of the carboxylate group.[125−127] The presence of the corresponding
Δν value >200 and <200 cm–1 in
the
same spectrum indicates both bidentate and monodentate −COO– coordination mode in the 2,3-pdca ligand which has
further been authenticated by X-ray single-crystal analyses (vide
supra). The other important absorption bands that appear in the IR
spectrum of 1 are νC–H (2980 cm–1) and νC=C (1464 cm–1) (Figure S19.1).[128] Peaks
appearing in the far-IR region indicate the formation of Zn–N
and Zn–O bonds.[129] Likewise, 2 exhibited vibration at 3517 cm–1 owing
to the coordinated water molecules along with characteristic vibrations
associated with νas and νs −COO– groups at 1661 and 1555 cm–1 and
at 1441 and 1379 cm–1, respectively (Figure S19.2).[124,125] Weak vibrations
present in the range 425–460 cm–1 may be
attributed to Cd–N and Cd–O stretching vibrations.[129] CP 3 shows vibrations at 3441
cm–1 (νH2O), 1583 cm–1 (νas–COO–), and 1369 cm–1 (νas–COO–) (Figure S19.3).[124,125,128,130] On the other hand, the FT-IR spectrum of 4 displayed
a vibration at 3238 cm–1 which may be attributed
to the −N–H stretching band of quaternary amine (CH3)2NH2+. Furthermore, characteristic
νas and νs bands of −COO– groups of 2,3-pdca appear at 1633 and 1592 cm–1 and at 1425 and 1382 cm–1 (Figure S19.4),[130] having
Δν value >200 cm–1 which indicates
that
the carboxylates of 2,3-pdca ligand are linked with Cu(II)/K(I) adopting
bidentate and/or bridged bidentate coordination modes.[129] Overall, the FT-IR data are consistent with
the structure obtained from SC-XRD analysis.
Powder
X-ray Diffraction Study
In
order to check the structural identity and phase purity of 1–4, powder X-ray diffraction (PXRD) analysis was performed at rt (Figure S20). The PXRD patterns of 1, 2, and 4 closely match with the patterns
simulated from their respective X-ray single-crystal diffraction data.
On the other hand, the major peak profiles of experimental PXRD of 3 closely match with experimental PXRD patterns of 1 and 2. It indicates that nclass="Chemical">CP 3 possesses
the structure very similar to those of 1 and 2. Notably, the PXRD class="Chemical">pattern of 4 varies from the rest
of class="Chemical">pan class="Chemical">CPs 1–3 in terms of the appearance of peaks
at a low 2θ angle (2θ = 8.12 corresponds to (2 0 0) reflection),
and a higher intensity ratio suggests a different and more porous
structural framework of 4. The relation of PXRD data
and transmission electron microscopy (TEM) image has also been discussed
in the Morphological Studies for 1–4 section (vide supra). The PXRD patterns of 1–4 recovered after catalytic reactions have also been obtained after
5 to 7 catalytic cycles (vide supra). Overall, the PXRD analyses of 1–4 are indicative of bulk sample purity.
Crystal Structure Studies
Structural
Description of 1 and 2
Solvothermal
reactions of nclass="Chemical">2,3-pdca with
Zn(NO3)2·6class="Chemical">pan class="Chemical">H2O and Cd(NO3)2·6H2O in the presence of KOH
afforded single crystals of [Zn(2,3-pdca)·(H2O)2] (1) and [Cd(2,3-pdca)(H2O)2] (2), respectively. CPs 1 and 2 are charge neutral 1D polymeric chains comprising fully deprotonated
ligands 2,3-pdca2– (Figure c,d). Complexes 1 and 2 assume a quite similar structure wherein the metals (Zn2+/Cd2+) act as a node and 2,3-pdca2– serve as a linker along with two coordinated water molecules on
each metal center (Figure a,b). Detailed crystallographic parameters for 1 and 2 have been listed in Table . X-ray single-crystal diffraction analysis
reveals that 1 and 2 crystallize in the
monoclinic P21/c space
group and the asymmetric unit consists of one Zn(II)/Cd(II), one 2,3-pdca2–, and two coordinated H2O [Figure a–d, Table ]. Each dianionic 2,3-pdca2– ligand was
linked with three metal centers adopting two distinct coordination
modes by N∧O chelating coordination to M(II) from one −COO– and M(II)–O–C–O–M(II)
bridging coordination from the other −COO– group [M = Zn(II)/Cd(II); Mode-XII-μ3-(κ4N,O2:O3:O3), Supporting Information]. Two coordination sites
have been occupied by two aqua ligands leading to a slightly distorted
octahedral geometry about the metal center with symmetry codes: (i) x, y, z + 1; (ii) −x + 1, −y + 1, −z + 1; and (iii) x, y, z – 1 for 1, whereas (i) −x + 1, −y + 1, −z +
1; (ii) x, y, z + 1; and (iii) x, y, z – 1 for 2. Furthermore, carboxyl oxygen atoms
are involved in intra- and intermolecular hydrogen-bonding interactions.
Figure 1
(a,b)
Asymmetric unit of 1 and 2; (c,d)
two types of cavities (square and ellipsoid shape) differing in dimensions
along the crystallographic “c”-axis.
Symmetry code for 1 (i) x, y, z + 1; (ii) −x + 1, −y + 1, −z + 1; and (iii) x, y, z – 1; for 2, (i) −x + 1, −y + 1, −z + 1; (ii) x, y, z + 1; and (iii) x, y, z – 1.
Table 1
Crystal Data and Structure Refinements
for 1, 2, and 4
compound
1
2
4
formula
C7H7NO6Zn
C14H14Cd2N2O12
C14H12CuKN3O8
formula
weight
266.53
627.09
452.91
crystal system
monoclinic
monoclinic
orthorhombic
space
group
P121/c1
P121/c1
Pnma
a (Å)
7.7081(4)
7.8597(5)
21.7641(13)
b (Å)
15.6681(7)
15.9234(10)
17.4903(10)
c (Å)
7.7483(4)
8.0981(5)
9.3685(5)
α (deg)
90
90
90
β (deg)
113.824(2)
114.849(2)
90
γ (deg)
90
90
90
V (Å3)
856.04(8)
919.67(10)
3566.2(4)
Z
4
18
8
Dcalc (g cm–3)
2.0679
2.2644
1.687
μ (mm–1)
2.879
2.383
1.51
F(000)
537.5656
605.1382
1832
θmin, θmax (deg)
2.87, 30.55
2.77, 30.57
25.7, 2.3
hmin–max
–11, 10
–11, 11
–26, 26
kmin–max
–22, 22
–22, 22
–21,
21
lmin–max
–10, 11
–11, 11
–11, 11
reflections collected
2611
2824
3503
data/restraints/parameters
2611/0/163
2824/0/62
3503/594/286
R1, wR2 [I > 2σ(I)]a
0.026866/
0.037138/
0.119709
R1, wR2 (all
data)a
0.037753/0.094958
0.041446/0.127273
0.1638/0.2423
no. unique data
2611
2824
3503
no. Observed
2241
2585
2320
no. Variables
163
62
286
Rint
0.0561
0.0388
0.229
wR
0.094958
0.127273
0.2423
GOF on F2
0.760475
1.078909
1.094
(Δρ)max,min (e/Å3)
0.742389(−0.645688)
2.328405(−4.14180)
2.04(−1.15)
(Δ/δ)max, (Δ/δ)mean CCDC
0.0003, 0.0000
0.0292,
0.0013
0.039, 0.000
(a,b)
Asymmetric unit of 1 and 2; (c,d)
two types of cavities (square and ellipsoid shape) differing in dimensions
along the crystallographic “c”-axis.
Symmetry code for 1 (i) x, y, z + 1; (ii) −x + 1, −y + 1, −z + 1; and (iii) x, y, z – 1; for 2, (i) −x + 1, −y + 1, −z + 1; (ii) x, y, z + 1; and (iii) x, y, z – 1.One of the coordinated nclass="Chemical">water molecules occupies an
apical position
with a longer Zn–O/Cd–O distance of 2.1651 Å/2.335
Å, whereas the other one assumes an equatorial position with
a shorter Zn–O/Cd–O distance of 2.0065 Å/2.226
Å. One class="Chemical">pan class="Chemical">oxygen atom from −COO– and one
pyridylnitrogen atom from 2,3-pdca ligand chelate to one Zn(II)/Cd(II)metal having Zn–N/Cd–N and Zn–O/Cd–O bond
distances 2.104 Å/2.313 Å and 2.056 Å/2.248 Å,
respectively. The transoid angles around Zn(II)/Cd(II) lie in the
range of 173.61–176.00°/169.24–176.44°, whereas
cisoid angles range from 77.74–99.76° to 72.89–90.77°
in 1 and 2 (Tables S13.1.3 and S13.2.2). Two types of M(II)···M(II) distances
occur in a double-stranded 1D chain of 1/2; (i) the distance between M(II)···M(II) separated
by bridging oxygen of the −COO– is uniformly
3.444 Å/3.629 Å and (ii) the M(II)···M(II)
separated by pdca2– ligands is 6.433 Å/6.453
Å (M = Zn, Cd; Schemes and 3, Figures b and 2b). The structure
of 1/2 reveals the presence of two sorts
of cavities in their polymeric chain; square shape cavities having
a dimension of 3.44 × 2.77 × 5.039 Å3/3.629
× 2.989 × 5.171 Å3 are created by two nearest
M(II) centers separated by V-shaped bridged oxygens. The dimension
has been measured by considering M(II)···M(II) and
O···O distances. Ellipsoidal cavities of dimension
(6.433 × 6.193 × 4.724 Å3/6.453 × 6.929
× 4.836 Å3 in 1/2)
are formed between M(II)···M(II) centers separated
by 2,3-pdca2– in the double-stranded chains and
the dimension has been calculated by distances between M(II)···M(II)
and O···O atoms, centroid–centroid distance
of the aromatic rings of bridged 2,3-pdca2– ligands.
In earlier reported CPs, Zn(II)···Zn(II) centers separated
by bridging oxygen of the −COO– is uniformly
3.294 Å and (ii) the Zn(II)···Zn(II) separated
by pdca2– ligands is 8.479 Å.
Scheme 3
Metal–Metal
Distances in Two Types of Cavities Present in
the Polymeric Double-Stranded Chain of 1 and 2
Figure 2
(a) Asymmetric
unit of 4 and (b) dimeric unit of 4. Symmetry
codes: (i) −x + 1, −y + 1, −z + 2; (ii) x, −y + 3/2, z; (iii) −x + 1, −y + 1, −z +
1; (iv) −x + 1, y + 1/2,
−z + 1; (v) x + 1/2, y, −z + 1/2; (vi) −x + 1/2, −y + 1, z – 1/2; (vii) −x + 1/2, y + 1/2, z – 1/2; (viii) x, −y + 1/2, z; (ix) x – 1/2, y, −z + 1/2; and (x) −x + 1/2,
−y + 1, z + 1/2. (c) Cap-stick
view of a cage-shaped 3D cavity present in 4 along the
crystallographic a-axis. (d) Demonstration of two
Cu···Cu distances between adjacent molecules of helical
and ellipsoidal cavities along crystallographic-b axes (hydrogen atoms are omitted for clarity).
Structural
Description of 4
The molecular structure of 4 was determined
by its X-ray single-crystal analysis (Figure ). Crystallographicparameters and selected bond lengths and bond angles for 4 have been listed in Tables and S13.3.1–S13.3.3, respectively.
nclass="Chemical">MOF 4 crystallizes in the orthorhombic crystal system
with the class="Chemical">pan class="Chemical">Pnma space group. The solvothermal reaction
of 2,3-pdca with Cu(II) in the presence of KOH followed by recrystallization
in dimethyl formamide (DMF) leads to the formation of a 3D network
{[(CH3)2NH2][CuK(2,3-pdca)(pa)(NO3)2]} (4) via mono-decarboxylation from one unit of 2,3-pdca to form picolinic
acid (PA–). By ignoring the disorder, an artificial
lowering of the bond lengths with disordered atoms and high values
of geometrical parameters has been observed. The counter cation (CH3)2NH2+ is present in the
cage to neutralize the negative charge present in the overall complex 4. The (CH3)2NH2+ entity resulted from the decomposition of DMF under solvothermal
conditions which is known in the literature.[131] Incorporation of (CH3)2NH2+ as a counter cation in an anionic coordination network has
previously been reported, although its source was not mentioned therein.[132−134] Interestingly, the single-crystal structure of 4 revealed
an unprecedented in situ decarboxylation of one carboxylate group
from 2,3-pdca2– to produce picolinic acid (HPA).
However, mono-decarboxylation occurred only from one unit of coordinated
2,3-pdca2– out of its two units linked with metal
ions (Figure b,d).
The Cu(II) centers assume distorted square pyramidal geometry completed
by N2O3 coordination sites offered by 2,3-pdca2– and PA– ligands wherein 2,3-pdca2– adopts two distinct coordination environments; (i)
one N∧O chelating site which also bridges through carboxylateoxygen and (ii) terminal coordination through 3-carboxylate. At the
same time, PA– coordinated through the N∧O
chelating mode to the Cu(II) center. The apical Cu–O bond distance
of 2.246 Å is slightly longer relative to the basal Cu–O
bonds. The Cu–N and Cu–O bond distances in the basal
plane lie in the ranges of 1.966–1.978 and 1.957–2.245
Å, respectively, which are comparable to the analogous systems.[131,135−139] Carboxylate groups and the pyridine ring are almost coplanar in
2,3-pdca. The adjacent distances between Cu(II)···Cu(II)
separated by square and ellipsoid shape cavities in the 1D polymeric
chain are 3.798 and 6.408 Å, respectively. A local coordination
environment with about seven coordinated K(I) centers can be best
described as a distorted tetragonal antiprism which consists of two
2,3-pdca2–, two PA–, and one nitrate
ligand. The coordination mode adopted by the 2,3-pdca2– ligand in 4 can be classified as Mode μ4-(κ7N,O2:O2,O2′:O2′,O3:O3′).
(a) Asymmetric
unit of 4 and (b) dimeric unit of 4. Symmetry
codes: (i) −x + 1, −y + 1, −z + 2; (ii) x, −y + 3/2, z; (iii) −x + 1, −y + 1, −z +
1; (iv) −x + 1, y + 1/2,
−z + 1; (v) x + 1/2, y, −z + 1/2; (vi) −x + 1/2, −y + 1, z – 1/2; (vii) −x + 1/2, y + 1/2, z – 1/2; (viii) x, −y + 1/2, z; (ix) x – 1/2, y, −z + 1/2; and (x) −x + 1/2,
−y + 1, z + 1/2. (c) Cap-stick
view of a cage-shaped 3D cavity present in 4 along the
crystallographic a-axis. (d) Demonstration of two
Cu···Cu distances panclass="Gene">between adjacent molecules of helical
and ellipsoidal cavities along crystallographic-b axes (class="Chemical">pan class="Chemical">hydrogen atoms are omitted for clarity).
The nclass="Chemical">polymeric 3D framework of 4 wherein class="Chemical">pan class="Chemical">2,3-pdca displays
an unprecedented μ7-coordination mode further extends
through intermolecular contacts. Another PA– ligand
assumes coordination mode Mode-XXV μ2-(κ3N,O2:O2′). The
K1–O distances are lying in the range of 2.670–2.941
Å, whereas K2–O distances range from 2.633 to 3.330 Å.
Several ellipsoidal cavities with dimensions (7.437 × 7.830 ×
11.872 Å3), (7.437 × 7.830 × 11.474 Å3), (3.829 × 5.694 × 11.872 Å3),
(3.829 × 5.694 × 11.474 Å3), (11.872 ×
11.474 × 9.940 Å3), (6.408 × 6.078 ×
4.496 Å3), and (6.408 × 6.078 × 9.714 Å3) are present in 4 which have been measured by
diagonal distances between centroids and centers separated by 2,3-pdca2–/PA– ligands (Figures and 3, S12d and S14). The Cu(II)···Cu(II)
centers are separated by two ways with distances 3.798 and 6.408 Å.
The apparent 3D cages present in 4 with said dimensions
are occupied by (CH3)2NH2+ cations through weak bonding interactions.
Figure 3
Cap-stick view of 4 along the crystallographic a-axis.
Cap-stick view of 4 along the crystallographic a-axis.
Supramolecular Interactions
in 1, 2, and 4
The
weak interactions
through H···bonding, C–H···π,
and π···π interactions lead to interconnectivity
of double-stranded chains in 1 and 2, fabricating
2D layers in the bc-plane. Intermolecular O–H···O
nclass="Chemical">hydrogen-bonding interactions class="Chemical">pan class="Gene">between oxygen atoms of water molecule
and carboxylate groups lead to the construction of 2D repeated double-stranded
chains along the crystallographic b-axis (Figures S2 and S7b). The distance between Zn···Zn/Cd···Cd
centers separated by intermolecular hydrogen bonding is 5.393/5.430
Å. In 1, the inter- and intramolecular H···bonding
distances lie in the range of 2.666–2.744 and 2.616–2.932
Å, respectively, whereas in 2, intermolecular H···bonding
distances lie in the range of 2.663–2.744 Å. Notably,
intramolecular hydrogen bonding is not observed in 2.
Intermolecular C–H···π interactions form
zigzag infinite parallel chains with hexagonal cavities both in 1 and 2 lead to a 2D network along the crystallographic b-axis having a dimension of 3.857 × 4.177 × 6.804
Å3 measured between O(2)···O(2), O(4)···O(4),
and O(7)···O(7) in 1 (Figures S2–S4) and a dimension of 4.338 × 4.482
× 7.064 Å3 measured between O(3)···O(3),
O(5)···O(5), and O(1)···O(1) in 2 (Figures S9–S11). In spite
of a similar structural framework in 1 and 2, the intramolecular π···π stacking distances
are lightly different [3.220, 3.243, and 3.373 Å in 1 and 3.354 and 3.391 Å in 2] while none of these
entails intermolecular π···π stacking interactions.
The centroid–centroid distances between two parallel aromatic
rings of bridging pdca2– are 4.725 and 4.836 Å,
respectively, in 1 and 2, whereas centroid–centroid
distances between intra- and interchain six-membered aromatic rings
are 4.724 and 5.372 Å in 1 and 4.836 and 5.486 Å
in 2. The nearest Zn···Zn separation in 1 is 3.444, whereas in 2, the adjacent Cd···Cd
distance is 3.629 Å (Scheme ). Slightly longer distances in 2 may
be due to larger ionic radii of Cd(II) in comparison with Zn(II).
Distances between μO(1)···μO(1) in 1 and 2 are 2.777 and 2.989 Å, respectively.
Notably, intra-/intermolecular H···bonding interactions
involve all oxygen atoms of 1 and 2 which
contribute significantly toward stability of intermolecular chains
(Table ). Prominently,
these compounds contain both the Lewis basic (the noncoordinated oxygen
atoms) and acidic centers (coordinated Zn(II)/Cd(II) ions) and coordinated
water molecules as the potentially good leaving group in the same
unit that enables these systems to find potential application as a
bifunctional catalyst (Figure ). On the other hand, supramolecular assemblies in 4 through intermolecular N–H···O (2.827 Å)
weak interactions between 2,3-pdca2– carboxylateoxygen and N–H of the dimethyl ammonium cation lead to 3D extensive
chains (Figure S18). The intermolecular
C–H···π distances were observed in the
range of 2.473–2.879 Å (Figure S13), whereas intermolecular π···π stacking
interaction distances occurring between pyridine rings of 2,3-pdca2– are observed to be 3.343 Å (Figure S17).
Table 2
Selected Hydrogen
Bond Geometry (Å)
in 1, 2, and 4
D–H···A
D···H
H···A
D···A
∠DHA
1
O7-Hc···O6
0.82(3)
1.82(3)
2.6149(18)
165(3)
O7-Hd···O6
0.82(3)
1.92(3)
2.7451(19)
177(3)
O4-Ha···O5
0.75(3)
2.03(3)
2.7749(19)
173(3)
O4-Hb···O2
0.82(3)
1.85(3)
2.6658(17)
173(3)
2
O5-Ha···O4
0.8700
2.03(2)
2.753(3)
140(3)
O1-H1a···O6
0.8700
1.895(9)
2.748(3)
166(3)
O1-H1b···O6
0.8700
1.913(7)
2.668(3)
144.4(9)
4
N1S-H1Sd···O6
0.92
1.939(14)
2.830(14)
162.4(4)
N1S-H1Se···O6
0.92
1.939(15)
2.830(14)
162.4(4)
Figure 4
(a) Cap-stick view of the 2D double-stranded chains incorporating
dimeric 1 between two layers. (b) Inter- and intramolecular
H···bonding interaction in 2 measured
from the crystallographic a-axis. (c) 3D view of 4, resulting from O–H···O hydrogen bonding
with a dimeric complex along the crystallographic b-axis.
(a) Cap-stick view of the 2D double-stranded chains incorporating
dimeric 1 panclass="Gene">between two layers. (b) Inter- and intramolecular
H···bonding interaction in 2 measured
from the crystallographic a-axis. (c) 3D view of 4, resulting from O–H···O class="Chemical">pan class="Chemical">hydrogen bonding
with a dimeric complex along the crystallographic b-axis.
Morphological Studies for 1–4
The morphological information was acquired for straightforward
comparison of surface structures of nclass="Chemical">CPs/class="Chemical">pan class="Chemical">MOF 1–4 so as to rationalize their distinct optical and catalytic behavior.
Shape and microstructure of the resulting 1–4 have
been investigated via scanning electron microscopy (SEM) (Figure top; Figures S25–S28). To have deeper insights,
SEM, TEM, and atomic force microscopy (AFM) analyses for 1–4 have been comprehensively illustrated. SEM, TEM, and AFM analyses
revealed fairly appealing and idiosyncratic external morphological
behavior by 1–4 (Figures and 6). SEM images
of 1–4 display homogeneous nanocrystallites aggregated
with an excellent pore size in the clusters. CPs 1–4 are shown as crystalline materials through SEM images with an elongated
block shape and particle sizes of about 30, 3, and 15 μm for 1–3, respectively. The morphological view of 1 displays interconnected crystalline fluffy (feathery) layered
sheets (Figure , top
1), whereas SEM images of 2 show elongated unruffled
nanorods with an approximate individual length ranging from 300 to
500 nm having uniform diameters of 20–40 nm. Moreover, SEM
images of 3 exhibit little bent uniform layer with obvious
small pores. Notably, 2 displayed an irregularly layered
fibrous array with a larger pore size relative to that of 1 (Figure , top 2).
In sharp contrast, SEM analysis of 3D MOF 4 exhibited
flower-like porous morphology with granular clusters (Figure , top 4). The hierarchical
structures in 1–4 are bound to possess a larger
specific surface area capable of absorbing substrate(s) or facilitating
heterogeneous catalytic activity to enable these materials as catalysts.
In addition, the EDX analyses reveal the presence of C, N, O, and
M with the respective weight ratio of 35.4, 10.3, 31.2, and 23.1 (M
= Zn, 1), 37.2, 16.4, 39.5, and 6.9 (M = Cd, 2), and 32.8, 8.9, 41.4, and 16.9 (M = Co, 3), whereas
C, N, O, K, and Cu elements with the respective weight ratio of 29.3,
37.2, 29.5, 7.8, and 26.2 (4) (Figures S29–S32). Overall, the EDX analysis strongly supports
successful synthesis of 1–4.
Figure 5
SEM (top) and HRTEM (bottom)
images of 1–4.
Figure 6
AFM images of 1 (a,e), 2 (b,f), 3 (c,g), and 4 (d,h).
SEM (top) and HRTEM (bottom)
images of 1–4.In addition, TEM analysis of thin films of 1 and 2 revealed typical flat sheet morphologies and apparently
visual cavities (Figure bottom 1 and 2). Interestingly, despite structural resemblance,
the TEM images exhibited different porous surfaces of 1 and 2 along with considerable liaisons among the arrays.
The HRTEM image of 1 exhibited the interplanar d-spacing of 0.705 and 0.392 nm which corresponds to the
respective (1 0 0) and (0 4 0) lattice plane of a monoclinic unit
cell of [Zn(nclass="Chemical">pdca)·(class="Chemical">pan class="Chemical">H2O)2]. The origin of these fringes in the HRTEM images is clearly
related to the PXRD data of 1. In the PXRD analysis of 1 (Figure bottom 1), major diffraction peaks at 2θ values are 11.286,
12.543, 16.904, 17.757, 21.113, 21.152, 21.798, 22.683, 23.937, 25.969,
27.530, 31.209, 32.577, 33.023, 34.313, 34.518, 34.593, 35.484, 36.669,
36.702, 38.144, 40.075, 42.234, 42.311, and 43.456 which may be assigned
to the reflection of (0 2 0), (1 0 0), (−1 2 0), (−1
2 1), (0 −3 1), (−1 3 0), (1 −1 1), (0 4 0),
(1 −2 1), (0 −4 1), (−2 0 2), (−1 5 0),
(−1 −4 2), (1 −1 2), (0 6 0), (1 −2 2),
(2 −2 1), (−3 1 1), (−1 6 0), (−1 −2
3), (−3 2 2), (2 −4 1), (−3 1 3), (0 −7
1), and (−3 2 3) crystal planes (Table S14.1). Likewise, the HRTEM image of 2 displayed
crystalline nanorod-shaped crystals and exhibited fringes having the
interplanar d-spacing of 0.367 nm corresponding to
the (0 0 2) lattice plane of a monoclinic unit cell of [Cd(pdca)·(H2O)2] (Figure , bottom 2). The positions
and relative PXRD peaks of 2 at around 2θ = 11.104,
12.401, 13.259, 13.593, 17.253, 20.845, 24.204, 25.426, 26.505, 27.785,
29.545, 30.721, 31.712, 32.018, 33.717, 35.050, 35.950, 36.367, 37.100,
37.845, 38.419, 39.488, 40.639, 41.859, and 43.62 which may be apportioned
to the reflection of (0 2 0), (1 0 0), (0 −1 1), (−1
1 0), (−1 2 1), (−1 3 0), (0 0 2), (−0 −4
1), (−2 0 2), (−1.–3 2), (0 −3 2), (−1
5 0), (1 0 2), (−2 4 1), (−2 4 0), (1 −5 1),
(0 −6 1), (−1 6 1), (−3 2 2), (−2 5 0),
(−0 −2 3), (2 −4 1), (−3 1 3), (−3
2 3), and (2 −2 2) crystalline lattice planes, respectively
(Table S14.2). The CP 3 comprises
small particle and crystalline morphologies in its HRTEM images which
shows an interplanar d-spacing of 0.4068 nm consistent
to the (1 1 1) lattice plane of a monoclinic system (Figure bottom 3). Positions and the
relative PXRD diffraction peaks of 3 are more or less
matching with those of 1 and 2. The positions
and the corresponding PXRD peaks of 3 at about 2θ
= 12.187, 13.366, 14.038, 14.687, 17.061, 17.437, 20.425, 21.895,
23.155, 23.254, 24.392, 24.515, 25.576, 29.363, 29.392, 29.622, 30.361,
30.438, 30.517, 30.921, 33.446, 34.260, 34.358, 35.271, 35.294, 35.588,
37.139, 37.242, 38.539, 38.828, 38.894, 39.016, 39.195, 39.776, 39.819,
39.837, 40.521, 40.585, 41.129, 41.365, 41.538, 42.261, 42.263, 43.481,
43.595, 43.738, 43.790, and 43.803 may be ascribed to the reflection
of (−1 0 1), (1 0 1), (0 −1 1), (−1 1 0), (0
0 2), (1 −1 1), (0 −1 2), (−1 −1 2), (−2
1 1), (1 −1 2), (−1 2 0), (−2 0 2), (−1
2 1), (−3 0 1), (−1 −2 2), (−2 2 0), (−2
2 1), (1 −2 2), (1 −1 3), (3 0 1), (−2 2 2),
(−3 1 2), (0 −2 3), (−1 3 0), (2 −2 2),
(2 −1 3), (−3 0 3), (−3 2 1), (1 −1 4),
(4 0 0), (−3 1 3), (−1 −3 2), (−2 3 0),
(−2 3 1), (−2 −1 4), (1 −3 2), (−4
1 0), (2 −3 1), (−4 0 2), (2 0 4), (−2 3 2),
(4 −1 1), (1 −2 4), (−1 −3 3), (−1
0 5), (2 −3 2), and (−3 2 3). Notably, the HRTEM image
of 4 shows a more compact surface which may be ascribed
to the lattice (CH3)2NH2+ cations present in the cavities. HRTEM images of 4 exhibit
the interplanar d-spacing of 0.386 nm which corresponds
to the (2 2 2) lattice plane of an orthorhombic unit cell of {[(CH3)2NH2][CuK(2,3-pdca)(pa)(NO3)2]} (Figure bottom 4). The PXRD pattern of the 4 having 2θ = 8.118, 9.564, 10.107, 10.704, 11.451,
12.977, 14.429, 16.278, 18.845, 20.293, 20.627, 21.249, 21.503, 23.019,
23.079, 24.521, 24.702, 24.725, 24.79, 25.065, 25.584, 25.596, 27.19,
27.261, 27.32, 27.502, 27.832, 27.89, 27.959, 29.435, 29.74, 29.80,
29.894, 30.185, 30.263, 30.483, 30.529, 30.537, 30.645, 30.651, 30.702,
32.033, 32.175, 32.401, 32.461, 32.499, 34.643, 36.134, 36.855, 37.472,
38.35, 41.265, 41.959, 41.963, 42.596, 45.129, 48.779, and 49.715
which may be attributed to the reflection of (2 0 0), (2 1 0), (0
2 0), (0 1 1), (1 1 1), (2 2 0), (1 2 1), (4 0 0), (4 0 1), (0 4 0),
(2 0 2), (2 1 2), (0 2 2), (2 2 2), (5 1 1), (6 0 0), (1 3 2), (5
2 1), (3 2 2), (4 0 2), (4 1 2), (3 4 1), (0 5 1), (5 3 1), (3 3 2),
(1 5 1), (4 4 1), (0 4 2), (5 0 2), (4 3 2), (2 0 3), (5 2 2), (3
5 1), (2 1 3), (7 0 1), (5 4 1), (6 3 1), (3 4 2), (0 6 0), (1 2 3),
(7 1 1), (6 4 0), (1 5 2), (1 6 1), (4 4 2), (0 3 3), (4 2 3), (7
2 2), (5 2 3), (6 4 2), (6 1 3), (7 1 3), (5 6 2), (4 0 4), (1 6 3),
(5 2 4), (5 4 4), and (4 5 4) lattice planes of the nanocrystallites
confirm well with its structure (Table S14.4).
AFM studies were also pursued to quantify the minimum and
maximum
values of precipitate size and accumulation pattern of 1–4. Momentarily, the surface roughness of the samples was characterized
by AFM and exnclass="Chemical">amined following the existing literature procedure.[140−142] The AFM image of 1 reveals uniformly class="Chemical">packed polycrystals
with a size of the crystals in the range of ∼30–50 μm
(Figure a,e). Moreover, the micelle-like morphology specifies
that 1 has a tendency to self-aggregate and further assemble
into the crystalline structures. The CP 2 exhibits class="Chemical">pan class="Chemical">polymeric,
spherical, granular, and porous domains which are clearly visible
in the AFM image. These porous and granular structures may provide
an increased surface area in 2 for catalytic applications
(Figure b,f). Furthermore,
the AFM image of 3 exhibits the formation of a well-established
crystalline material which gets converted into twisted ciliated fiber-like
morphology spread over the film (Figure c,g). The AFM image of 4 demonstrates
roughly porous and shaggy layered morphologies having a very small
uniform granule-like surface (Figure d,h). The 2D and 3D maps of 1–4 are also given in Figure which indicate different surface morphologies. Average roughness
(Ra) obtained from the films of 1–4 is 0.28, 2.52, 0.32, and 2.06 nm, respectively.
Moreover, section analysis of the 2D images across the line in 1–4 affords the average tube diameter of around 12–15
nm (Figures S37–S41). The root mean
square (rms) roughness (Rq) has been calculated
to be 0.332, 2.71, 0.328, and 2.17 nm for 1–4,
respectively (Figure S41). The low-to-high
values in rms roughness are probably ascribed to the noncovalent intermolecular
interactions on the surfaces of the polymers 1–4.[143−146] As Rq is more sensitive to the peak
and valley relative to that of Ra, typically Rq is 3 to 15% higher than Ra in these materials. A smaller roughness is associated
with greater density and finer precipitates, whereas larger precipitates
tend to increase the roughness value (Ra = 0.28–2.52 nm in 1–4). Overall, the
SEM, TEM, and AFM analyses strongly suggest diverse structural morphology
and porous surface of 1–4. The different intermolecular
interactions may be responsible for morphological disparities in 1–4.
AFM images of 1 (a,e), 2 (b,f), 3 (c,g), and 4 (d,h).
UV/Vis and Luminescent Spectral Analyses
Electronic absorption spectra for the suspended particles of 1–4 have been recorded in nclass="Chemical">DMF (Figure S22). class="Chemical">pan class="Chemical">CPs 1 and 2 comprising
diamagnetic Zn(II) and Cd(II)metals exhibited broad absorption bands
at 286 and 287 nm, whereas 3 and 4 involving
paramagnetic Co(II) and Cu(II)metals displayed bands having the absorption
maximum around 289 nm. Therefore, observed absorption bands for 1–4 may be assigned to the ligand-centered transitions.
It has been well-established that d10-transition metal
complexes reveal fascinating luminescence properties.[147,148] Hence, the solid-state photoluminescent behavior of 1–4 was also investigated at rt. CPs 1 and 2 encompassing d10-metal centers and conjugated organic
ligands were expected to serve as facilitating inorganic–organic
hybrids for prospective application as chemical sensors and in photochemistry.[149−153] Notably, 1–4 exhibited moderate luminescence
in the solid state.[154] CPs 1–3 showed emission spectra having maxima at 398, 390, and 385 nm along
with a shoulder peak at 421, 414, and 422 nm, respectively, upon excitation
at their respective wavelengths (286–289 nm), whereas 4 displayed an emission band at 387 nm (λex, 289) along with a shoulder peak at 424 nm, respectively (Figure S23). The emission of 1–4 may be ascribed to the ligand-centered transitions and/or ligand-to-metal
charge-transfer transitions.[118,155−160]
TGA and DSC Analysis of 1–4
To further assess the thermal stability of 1–4, thermogravimetric analysis (nclass="Gene">TGA) was performed on crystalline samples
from 40 to 800 °C under a class="Chemical">pan class="Chemical">nitrogen atmosphere at a heating rate
of 10 °C/min (Figures a and S21). A weight loss of 8.49%
until 132 °C followed by another weight loss of 7.37% until 228
°C was observed for 1 which corresponds to the release
of trapped and coordinated solvent molecules (calculated 13.54%).
The loss of coordinated water molecules in 2 was completed
by 169 °C with a weight loss of 9.38% (calculated 11.48%). In 3, a weight loss of 14.23% was occurred between 40 and 133
°C which is consistent with the loss of coordinated water molecules
(calculated 13.84%). The rest of the framework remains thermally stable
up to ∼300 °C; thereafter, the desolvated framework starts
decomposing the organic components and completes by ∼460, ∼417,
and ∼430 °C for 1–3, respectively.
Compounds 1–3 exhibit similar decomposition processes
which corroborate well with their structures (SCXRD and PXRD). Notably,
the thermogravimetric histogram of the 4 does not exhibit
any inflexion point before ∼216 °C, which indicates its
good thermal stability and strongly supports the lack of water molecule(s)
[crystalliferous, lattice, or coordinated (Figures a and S21)]. Exceeding
220 °C, it was sharply decomposed by losing almost entire organic
2,3-pdca2– ligands with a weight loss (wt loss)
of 53.99%. The remaining dark-brown scorched compound (unidentified)
is obtained at 800 °C, which may be the metal oxides or metalcarbonates. Overall, TGA analysis indicates good thermal stability
of 1–3 and excellent thermal stability of 4.
Figure 7
TGA (a) and DSC (b) profile of 1–4.
panclass="Gene">TGA (a) and DSC (b) profile of 1–4.
In addition, DSCcurves for 1–3 exhibited
endothermic
peaks followed by exothermic peaks (collapse/desolvation) of the framework
(Figures b and S22). The values of ΔHf (J/g) and ΔSf were
calculated from DSC endothermic peaks (nclass="Chemical">DTGmax = 128 °C;
ΔH = +181.2 J/g, ΔSf = 1.42 and class="Chemical">pan class="Chemical">DTGmax = 210.8 °C; ΔH = +185.8; ΔSf = 0.88;
DTGmax = 320 °C; ΔH = 21.36
J/g, ΔSf = 0.067) for the first
stage while an exothermic peak (DTGmax = 378.18, 552.49
°C, ΔHm = −1687 J/g,
ΔSf = −3.63) for the second
stage for 1. In contrast, 2 displays an
endothermic peak (DTGmax = 165 °C; ΔHf = +154.9 J/g; ΔSf = 0.94) in the first stage and an exothermic peak (DTGmax = 388.93, 569.13 °C, ΔHm = −4118 J/g; ΔSf = −8.59) in the second stage. On the other hand, ΔHf and ΔSf for 3 have been calculated from endothermic peaks (DTGmax = 174.9, 366.7, and 519 °C; ΔH = 27.85,
65.83, and 36.63 J/g; ΔSf = 0.16,
0.18, and 0.071) in the first stage and (DTGmax = 337.80,
452.55, and 542.58 °C, ΔHm −730.6
J/g, ΔSf = −1.64) in the
second stage. Additionally, 4 displayed a sharp endothermic
peak at 257.7 °C followed by breakdown of the framework. The
ΔHf and ΔSf for 4 were calculated from an endothermic
peak (DTGmax = 257.7 °C; ΔH = +198.6 J/g, ΔSf = 0.77) for
the first stage while in the second step, DTGmax = 401.16
and 564.59 °C, ΔH = −3015 J/g;
ΔSf = −6.24. Overall, the
DSC studies suggest a better stability of 4 over 1–3, whereas 1–3 follow the stability
order 3 > 2 > 1 (Figures b and S22). The
greater
stability of 4 over 1–3 is also associated
with the absence of coordinated water molecules in 4.
Moreover, intermolecular associations may be the major factor of stability
order 3 > 2 > 1.
While correlating DSC studies
with the crystal structure, it can
be stated that the framework structure converts from stable six-coordinated
distorted Oh to less-stable four-coordinated Td geometry upon removal of two nclass="Chemical">water molecules; hence, the process
is endothermic in 1–3. Eventually, the remaining
dark-brown scorched compound (unidentified) obtained at 800 °C
may be the more-stable class="Chemical">pan class="Chemical">metal oxides or metal carbonates; hence, this
process is exothermic. On the other hand, DSC of 4 shows
one endothermic peak at 257.7 °C which may be attributed to the
conversion of more-stable to less-stable species upon removal of lattice
solvent/guest molecule followed by exothermic breakdown of the framework
to attain metal oxides/carbonates.
Catalytic
Studies on 1–4
The lability of ligands
pronclass="Chemical">vides coordinately unsaturated
active catalytic centers to carry out various catalytic reactions.
The Knoevenagel condensation and C–H activation reactions are
one of the most studied heterogeneous catalytic reactions performed
for porous and framework materials of various class="Chemical">pan class="Chemical">metal centers, that
is, Co, Cu, Zn, Ag, Cd, In, and so forth. The PCPs connected to several
flexible ligands containing distinct aromatic rings are appropriate
scaffolds for such catalytic reactions because metal centers exhibit
variable oxidation states and coordination numbers.[161] The Knoevenagel reaction is well-known not only as a weak
base-catalyzed model reaction but also as a C–C bond formation
reaction.[89,162−166] In the present work, catalytic synthesis of 2-benzylidenemalononitrile
has been chosen mainly because these intermediates are often used
in polymers, perfumes, cosmetics, fine chemicals, pharmaceuticals,
and drugs.[141,182,185−191]
Activation Method
Catalysts 1–4 were heated in an oven at 100 °C for 24 h
under vacuum and a N2 atmosphere to remove the air/solvent
molecules from the pores of panclass="Chemical">CPs. The heating temperature was selected
considering thermal analyses so as to avoid structural class="Chemical">pan class="Disease">rupture.
Knoevenagel Condensation Reaction Catalyzed
by 1–4
The optimized activated catalysts 1–4 (5.0 mol %) were added in the reaction mixture
of nclass="Chemical">benzaldehyde (1.0 mmol) and class="Chemical">pan class="Chemical">malononitrile (1.1 mmol) and the resulting
mixture was stirred at rt for a different time scale depending upon
progress of the reaction monitored by thin layer chromatography (TLC).
Information on the product formation was analyzed using 1H and 13C NMR and FT-IR techniques (Figures S42–S46). Upon reaction completion, the mixture
was diluted with 5 mL of CH3OH to dissolve the organic
products which was followed by centrifugation cum filtration and the supernatant liquid was evaporated to dryness.
The pure product was obtained by recrystallization in MeOH. The catalyst
was removed by filtration and washed with EtOAc which was recovered,
dried, and reused as required (Scheme ). 2-Benzylidenemalononitrile (Table , entry 1) mp: 83.5 °C; 1H NMR (500 MHz, CDCl3; δ, ppm): 7.89 (d, J = 8.0 Hz, 2H), 7.78 (s, 1H), 7.63 (t, J = 7.5 Hz, 1H), 7.54 (t, J = 7.8 Hz, 2H). 13C NMR (125 MHz, CDCl3; δ, ppm): 160.03, 134.76,
131.01, 130.83, 129.68, 113.83, 112.67, 82.93. FT-IR (KBr, cm–1): 3426, 3033, 2956, 2867, 2777, 1695, 1668, 1592,
1569, 1491, 1461, 1375, 1336, 1300, 1231, 1202, 1182, 1163, 1144,
1101, 1001, 972, 776, 756.
Scheme 4
Synthesis of 2-Benzylidenemalononitrile
Using Heterogeneous Catalysts 1–4
Optimization of the Catalyst and Solvent
for the Knoevenagel Condensation Reaction
The objective of
these optimizations was to develop economical and most effective catalyst
and solvent system for the Knoevenagel reaction. To seek for the possibility
of heterogeneous catalytic behavior of 1–4, the
one-pot Knoevenagel reaction was carried out in different optimized
solvents. At the onset, catalysts 1–4 were separately
employed in different sets of the model Knoevenagel reaction using
nclass="Chemical">benzaldehyde and class="Chemical">pan class="Chemical">malononitrile reactants to yield 2-benzylidenemalononitrile
at rt under solvent-free conditions. The reaction process can be constantly
monitored by TLC, illustrating that this catalytic reaction indicated
a significant amount of product after 25 min. Taking a random amount
of catalysts for the fixed timescale of 25 min, 1–4 provided the product yields 61, 68, 59, and 53%, respectively. Notably,
the said reaction completes in 4 h under catalyst-free conditions.
To select the most appropriate reaction condition, the parameters
such as catalyst amount and best solvent were optimized (Tables S1–S4). The performance of 1–4 was examined by varying the catalytic amount to
1, 2, 3, 4, 5, and 10 mol % in the reaction and observed that increasing
the catalytic loading from 1 to 5 mol % leads to the enhanced yield
of the product; however, escalating the amount from 5 to 10 mol %
could not significantly increase the product yield at the same time
scale. The kinetics of conversion profiles show that the initial reaction
rates and the final product yield were higher with 5 mol % relative
to the other catalytic loadings; hence, the best catalytic loading
of 1–4 chosen was 5 mol % (Figure S47, Tables S1–S4, entry 6). In addition, the
reaction was investigated with various solvents, that is, H2O, EtOH, CH2Cl2, CH3CN, toluene,
and benzene, and also under solvent-free conditions wherein moderate
yields were observed using these solvents (Tables S1–S4, entries 8–14), whereas solvent-free conditions
afforded excellent yields of the product in the 25 min time scale.
The lower yields in H2O and EtOH relative to that of the
solvent-free reaction may be attributed to the solvation of the active
functional groups by these solvents as well as due to hydrogen-bonding
interaction between active protonic sites of 1–4, thereby diminishing in the catalytic efficiency. Therefore, 5 mol
% catalytic loading of 1–4 under the solvent-free
conditions at rt was designated as the optimum condition (Tables S1–S4).
Among the tested
solvents, nclass="Chemical">ethanol has been observed as the best solvent for the Knovenegal
reaction; hence, further experiments for the optimization of catalyst
loading were performed in class="Chemical">pan class="Chemical">ethanol. A blank-controlled experiment afforded
38% conversion of benzaldehyde after 4 h in ethanol at
rt, whereas 95–98% conversion could be accomplished using catalysts 1–4 under the same conditions and 25 min time scale
(Figures S52 and S53). The time-conversion
plot exhibiting the comparative reaction rate in the presence and
absence of 1–4 clearly suggests that the reaction
is promoted only in the presence of catalysts (Figure S53). Furthermore, leaching experiments have also been
conducted under identical conditions to assure whether the reaction
is stimulated by the heterogeneous catalysts not due to the active
sites leached into the solution. Therefore, the reaction between benzaldehyde
and malononitrile was initiated in the presence of 1–4 under identical conditions and the solid catalyst was removed by
filtration after 10 min while the resulting solution was allowed to
continue up to 25 min (Figure S55). Notably,
the reaction rate was significantly reduced in the absence of a catalyst
which indicates that the reaction is exclusively catalyzed by 1–4. Comparison of the reaction rates for 1–4 clearly indicates a higher initial reaction rate for 2 relative to those of 1, 3, and 4 which may be attributed to either the lack of diffusion restrictions
or possibility of increasing coordination number for the Cd(II) center.
Furthermore, recovered catalysts have also been analyzed by PXRD which
revealed the structural and morphological stability of 1–4 (Figure ).
Figure 8
PXRD patterns
of 1–4 freshly prepared, recovered
after the first cycle, and recycled after seven (1–3) and five (4) cycles.
PXRD patterns
of 1–4 freshly prepared, recovered
after the first cycle, and recycled after seven (1–3) and five (4) cycles.Hence, the stability of 1–4 was exnclass="Chemical">amined by
recycling the catalyst in the successive runs up to seven cycles and
observed consistent performance for 1–3; however, 4 displayed consistent performance up to five cycles only
(Figure S50). Relatively lower reusability
observed for 4 may be attributed to the presence of class="Chemical">pan class="Chemical">K(I)
centers along with Cu(II) centers in the framework. Moreover, the
stability of these catalysts during the reusability experiments was
also ascertained by comparing the PXRD pattern of the fresh with those
of recovered catalysts up to the respective consistent performance
(Figure ). The catalytic
efficiency of a heterogeneous catalyst with respect to the previously
reported catalysts can be best assessed through calculation of turnover
number or turnover frequency; therefore, 1–4 have
been exhaustively compared with earlier reports. However, the yield
achieved in the presence of 1–4 at rt is preferable
over the catalysts which operate either at 60–130 °C or
involve large time scale reactions (12–24 h).[23a] High yield (98%), ambient temperature (rt), and shorter
time (4 h) along with catalyst stability up to many cycles demonstrate
the improvement of catalysts 1–4 relative to the
earlier reports (Table S11).[23b,62] The salient features of 1–4 comprise high performance
in the Knovenagel reaction under solvent-free conditions, functioning
under mild reaction conditions, short reaction time, wide substrate
scope, and high stability.
Plausible Reaction Mechanism
nclass="Chemical">CPs
can act as Lewis bases or acids in the catalytic medium depending
on the nature of the ligands and class="Chemical">pan class="Chemical">metal ions as well as the coordination
environment. In 1–4, metal centers Zn(II)/Cd(II)/Co(II)/Cu(II)
impart Lewis acidic sites and the carboxylate group present in the
2,3-pdca linker can act as a weak Bronsted base. Thus, 1–4 can be considered as a potential acid–base catalyst for the
Knoevenagel condensation reaction in which acidic and basic sites
can synergistically catalyze the reaction to improve its efficiency.
As observed in the SC-XRD data, the offset packing of the networks
in 1–3 perhaps blocks the channels supporting
the nonporous nature and hence, the condensation reaction may be occurring
on the catalyst surface. On the basis of crystal structure and based
on the previous reports,[75,167] a plausible mechanism
for the Knoevenagel reaction on the Lewis acidic site has been illustrated
(Scheme ). The reaction
is plausibly initiated by the attack of the polarized carbonyl oxygen
from the aldehyde with the metal center of the respective CP, followed
by subsequent opening of the weakly coordinated chelating carboxylateoxygen from the Lewis acid site. Simultaneously, detachment of an
acidic proton of the methylene group of the malononitrile produces
a carbonium anion. In the next step, the carbonium anion reacts with
the carbonyl group of benzaldehyde to give an intermediate which undergoes
rearrangement and elimination of water to afford the final product.
Scheme 5
Proposed Mechanism for the Knoevenagel Reaction, Catalyzed by 1–4
Recyclability
of Catalysts 1–4
Catalysts 1–4 might have pronclass="Chemical">vided lodgings
for activated guests in their channels, consequently, the prominent
Knoevenagel condensation reaction was selectively stimulated to afford
the product in good yield. To exclass="Chemical">pan class="Chemical">amine the recyclability of 1–4 upon reaction completion, the solid catalysts were collected and
separated dissolving the reaction mixture in ethanol followed by filtering
and drying to reuse for another set of the same reaction. After filtration
and drying, the catalysts were subjected to a vacuum oven at 80 °C
for 4–6 h to remove adsorbed guests. The vacuum-dried catalyst
was then reused for the next cycle of the same reaction. This experiment
was repeated seven times and found that 1–3 perform
quite well, whereas 4 starts decreasing in its catalytic
activity after five catalytic cycles (Figure ). The deactivation of the catalysts after
the seventh and fifth cycle may be attributed to the pore blocking
by the generated product.[178,179] Remarkably, PXRD patterns
of the recycled CPs 1–4 after the seventh and
fifth runs, respectively, exhibited peak profiles similar to those
of the fresh compounds (Figure ). Relative performances of 1–4 with some
previously reported catalysts for the Knoevenagel condensation suggested
that our catalysts are comparative in terms of cost-effectiveness,
ambient temperature, and short time scale of the reaction (Tables S11 and 3).[33,88,168,180,181]
Table 3
Knoevenagel
Reaction between Malononitrile
and Benzaldehyde Catalyzed by 1–4 in Ethanol
entry
catalyst (mol %)
optimized catalyst quantity
time (m)
yield
(%)
1
[Zn(pdca)·(H2O)2]
5 mol %
5
47
10
67
15
82
20
92
25
97
2
[Cd(pdca)·(H2O)2]
5 mol %
5
59
10
83
15
92
20
95
25
98
3
[Co(pdca)·(H2O)2]
5 mol %
5
53
10
69
15
83
20
93
25
96
4
[(CH3)2NH2][CuK(pdca)(PA)(NO3)2]
5 mol %
5
51
10
74
15
85
20
92
25
95
Hot-Filtration Experiments
To confirm
the heterogeneous nature of the catalysts, hot-filtration experiments
were also performed.[169−174] In this context, the solid catalysts 1–4 were
removed from a hot solution by filtration for 10 min after initiating
the catalytic run. The reaction of the filtrate was then monitored
for another 15 min wherein insignificant catalyticconversion was
observed (Figure S55). It indicates almost
no leaching of panclass="Chemical">metal from the catalyst and thus the heterogeneous
nature of the catalysts. The filtrate was also analyzed by atomic
absorption spectroscopy (AAS), which indicated a very low concentration
of free class="Chemical">pan class="Chemical">metal(II) ions (0.000017–0.000079%) from catalysts 1–4 that leached out into the reaction solution (Table S9).[175−177]
Synthesis of Benzoic Anhydrides via Aldehydic
C–H Activation Using Catalysts 1–4
To a solution of nclass="Chemical">benzaldehyde (1.0 mmol) and catalyst 1/2/3/4 (5 mol %) in class="Chemical">pan class="Chemical">CH3CN (2 mL), a 70% solution of TBHP (1.5 equiv) in CH3CN was added gradually over 10 min under stirring. Furthermore, the
reaction temperature was increased to 70 °C followed by stirring
for another 1 h under a N2 atmosphere and the progress
of the reaction was monitored by TLC (Scheme ). After reaction completion, the solvent
was evaporated under reduced pressure and the residue was purified
using silica gel column chromatography (hexane/EtOAc, 4.5:0.5). Benzoic
anhydride: Colorless crystalline powder; yield: 98%; Anal. Calcd for
C14H10O3 (226.23): C, 74.33; H, 4.46;
N, 0.0. Found: C, 74.36; H, 4.48, N, 0.0. R = 0.5797, 1H NMR (500 MHz, CDCl3): δ 8.13 (d, J = 7.5 Hz, 2H), 7.63 (t, J = 7.5 Hz, 1H), 7.50 (t, J = 7.8 Hz, 2H). 13C NMR (125 MHz, CDCl3): δ 162.48, 134.66,
130.67, 128.99, 128.59. IR (KBr, cm–1): ν
= 3438, 3065, 3036, 3011, 1789, 1725, 1686, 1650, 1599, 1493, 1452,
1345, 1314, 1279, 1173, 1098, 1076, 1049, 996, 870, 801, 778, 702.
Scheme 6
Syntheses of Anhydrides via Aldehydic C–H Bond Activation
Using Heterogeneous Catalysts 1–4
To employ 1–4 as heterogeneous catalysts
for
the production of panclass="Chemical">benzoic anhydrides via C–H bond activation,
the best reaction conditions and various other class="Chemical">parameters such as
solvent, oxidant, amount of catalyst, and reaction time were optimized.[182,183]
Optimizations for C–H Bond Activation
Reactions in the Presence of 1–4
The
effect of various solvents such as nclass="Chemical">CH2Cl2, class="Chemical">pan class="Chemical">ClCH2CH2Cl, chloroform, ethanol, water, diethyl ether,
THF, DMF, and CH3CN was investigated independently for
the C–H activation of benzaldehyde in the presence of the catalyst
and TBHP, as a model reaction (Table ). There are definite evidences on solvent-dependent
C–H bond activation of benzaldehyde. The results of the present
study revealed the maximum yield obtained using a CH3CN
solvent; therefore, it was chosen as an ideal solvent for this reaction
(Tables and 4).
Table 4
Solvent Optimizations
for the C–H
Bond Activation Reaction of Benzaldehyde Using 1–4a
Among
oxidants such as air, nclass="Chemical">TBHP, class="Chemical">pan class="Chemical">H2O2, and oxone,
TBHP was optimized as the best oxidant for the abovementioned reaction
(Table , entry 7).
It is worth mentioning that using H2O2 as an
oxidant leads to the conversion of a large part of aldehyde to carboxylic
acid and the negligible desired product was observed.
Table 5
Screening of Reaction Conditions and
Optimization of the Best Oxidanta
entry
oxidant used for desired product
% age yield
1
air/oxygen
N.O.
2
CHP
N.O.
3
oxone
N.O.
4
K2S2O8
N.O.
5
H2O2
16
6
TEMPO
N.O.
7
TBHP
83
Reaction conditions: benzaldehyde
(1 mmol), oxidant (1.5 equiv), and CH3CN (2 mL). N.O. =
not observed.
Reaction conditions: panclass="Chemical">benzaldehyde
(1 mmol), oxidant (1.5 equiv), and class="Chemical">pan class="Chemical">CH3CN (2 mL). N.O. =
not observed.
Effect of the Catalyst Quantity
The amount of the catalysts 1–4 was optimized
in panclass="Chemical">CH3CN in the presence of class="Chemical">pan class="Chemical">TBHP as an oxidant. The best
results obtained using 5 mol % of catalysts, whereas the exceeding
catalytic quantity does not show any noticeable enhancement in the
product yield (Table ).
Table 6
Optimization of Catalytic Amount of 1–4 in the Synthesis of Benzoic Anhydride in CH3CN at rta
Reaction conditions: nclass="Chemical">benzaldehyde
(1.0 mmol), class="Chemical">pan class="Chemical">TBHP (1.5 equiv), and CH3CN (2 mL).
After optimization of the model
reaction, generality of the reaction
was exnclass="Chemical">amined using class="Chemical">pan class="Chemical">benzaldehyde with both electron-rich and electron-poor
substituents and the results showed high catalyst proficiency for
C–H bond activation cum formation of anhydrides.
Reportedly, the aldehydes with a NO2 substituent at para-
or ortho-positions in the presence of CuO nanoparticles resulted in
the NO-product formation.[27] Other literature
reports illustrate the reaction of benzaldehyde in the presence of
CuO, CuCl, Cu-MOFs, CuCl2, and TBHP which yielded 35–75%
of the anhydrides, after 3 h[63,184] (Table S12, entries 6–8 and 10). Notably, in the presence
of 1–4, aldehydes having electron-withdrawing
groups lead to the formation of the corresponding anhydrides in relatively
good yields, whereas aldehydes with electron-donating groups were
converted to the respective anhydrides with good-to-excellent yields
(Table S12 entry 13–16). After reaction
completion, the catalysts were feasibly separated by filtration and
recycled several times. On the other hand, utmost of the reported
methods used an equivalent amount of reagents which are expensive
and environmentally alarming (Table S12, entry 1–5).
Catalyst Recycling of 1–4
Reusability with consistent performance
is one of the important
concerns in heterogeneous catalysts. Notably, 1–4 could be recovered simply by filtration, washing with panclass="Chemical">methanol,
and drying under a vacuum oven @ 80 °C for 4–6 h and these
recovered 1–4 were used for the same C–H
bond activation reaction up to four cycles. However, the results indicate
that 1–4 can be used for several cycles with minimal
loss of activity which is also eclass="Chemical">pan class="Chemical">vident from unchanged HRTEM structural
morphology even after seven catalytic cycles (Figure S58). The HRTEM images strongly suggested preserved
crystallinity as well as morphology even after seven catalytic cycles
of 1–3 and five catalytic cycles of 4. Likewise, no framework degradation of the hydrolysis reaction[63] could be significantly observed in the PXRD
patterns (Table ).
Table 7
Synthesis of Benzoic Anhydrides via
the C–H Activation of Benzaldehyde Catalyzed by 1–4 (5 mol %) in CH3CN
entry
catalyst
time (min)
yield (%)
1
[Zn(pdca)·(H2O)2]
2
38
5
56
10
77
15
89
20
97
2
[Cd(pdca)·(H2O)2]
2
42
5
61
10
87
15
94
20
99
3
[Co(pdca)·(H2O)2]
2
36
5
51
10
68
15
92
20
97
4
[(CH3)2NH2][CuK(pdca)(PA)(NO3)2]
2
41
5
53
10
71
15
93
20
98.5
Hot-Filtration Test
Furthermore,
leaching of nclass="Chemical">metal ions from the heterogeneous surface of catalysts 1–4 was exclass="Chemical">pan class="Chemical">amined through the hot-filtration test. Hence,
the reaction was initiated under the optimized reaction conditions
and the reaction mixture was filtered off to separate the solid catalysts
after 5 min under hot conditions. The filtrate in the absence of solid
was continued up to 20 min (Table S10).
The analysis of the reaction mixture indicated that the yield of the
reaction was insignificantly improved (Figure S56). It strongly indicated the heterogeneous nature of the
catalysts 1–4. Moreover, the filtrates were also
analyzed by AAS which exhibited a complete absence of free metal(II)
ions (0.000000–0.000000) indicating no leaching from catalysts 1–4 (Table S10).
It
is noteworthy to mention that the catalytic performance for the Knoevenagel
condensation reaction follows the order of 2 > 1 > 3 > 4. The literature
reveals
that many factors including the particle size, shape, chemical composition,
nclass="Chemical">metal–support interaction, and class="Chemical">pan class="Chemical">metal–reactant/solvent
interaction can have significant influences on the catalytic properties
of the catalysts. It is worth stating that since all the catalysts
comprise the same pdca2– ligand and 1–3 are isostructural, the difference in product yield is small (95–99%)
which may be attributable to the feasibility of basic sites, ionic-covalent
nature of the metal–oxygen bonds, ready production of “M–O2 Lewis acid–base”
pair, and so forth. All the metal centers [Zn(II), Cd(II), and Co(II)]
are six-coordinated and possess entirely the same coordination environment
by pdca2– and aqua ligands; therefore, small variation
in catalytic performance may be attributed to the size of the metal
ions and their electronic configuration. The ionic radii order for
these metal ions follows order Cd(II) > Zn(II) > Co(II) which
is analogous
to their catalytic performances. Therefore, better catalytic performance
of 2 over 1, 3, and 4 may be ascribed to the larger size of Cd(II) and its possibility
to extend coordination no up to 7-, 8-, and 9-coordination. On the
other hand, the catalytic efficiency order follows 2 > 4 > 1 > 3 for the C–H
activation
reaction which may be attributed to the rms roughness (Rq) which has been calculated to be 0.332, 2.71, 0.328,
and 2.17 nm for 1–4, respectively. In addition,
five-coordinated Cu(II) in 4 may offer a better binding
site for the substrate over six-coordinated metal centers in 1–3. Moreover, the catalytic activities of 1–4 may be ascribed to the combined effect of the metal atom involved
in the respective CP and/or porosity of the framework. The CPs 1–4 contain both Lewis acid (Zn, Cd, Co, and Cu center)
and Lewis basic (carboxylate and free pyridyl groups) sites, which
makes them suitable for bifunctional catalysis.
Experimental Section
Chemicals and Reagents
All the chemicals
including Zn(NO3)2·6nclass="Chemical">H2O, Cd(NO3)2·4class="Chemical">pan class="Chemical">H2O, Co(NO3)2·6H2O, Cu(NO3)2·3H2O ≥ 99.00%, and potassium hydroxide (KOH) were procured
from HiMedia Chemicals Pvt. Ltd., India. Rare chemical pyridine-2,3-dicarboxylic
acid or quinolinic acid was purchased from Sigma-Aldrich Pvt. Ltd.,
USA. All the solvents such as ethanol and distilled water were purchased
from Merck Life Science Pvt Ltd (India). The reagents were procured
from commercial sources and used as received. The chemicals and reagents
were pure, whereas the solvents were dried and distilled following
standard literature procedures.[123]
Conclusions
Four nclass="Chemical">CPs [M(class="Chemical">pan class="Chemical">pdca)·(H2O)2] [M = Zn(II),
Cd(II), Co(II)] (1–3), and [(CH3)2NH2][CuK(pdca)(pa)(NO3)2]
(4) have been synthesized implementing a solvothermal
reaction strategy. CPs 1–4 have been thoroughly
characterized using various spectral techniques, that is, elemental
analyses and thermal and spectral techniques. Moreover, the structural
information on 1–4 have been obtained by PXRD
and X-ray single-crystal analyses, whereas morphological insights
were acquired by FESEM, AFM, EDX, and HRTEM analyses and BET surface
area analysis. The roughness parameter was calculated from AFM, whereas
HRTEM aided in defining the dimensions of small domains and in measuring
the interplanar spacing in the CPs. The structurally similar 1–3 are 1D coordination frameworks, whereas 4 is a 3D network. In addition, all 1–4 display
good luminescence at rt. Furthermore, feasibly prepared 1–4 serve as efficient and economic porous heterogeneous catalysts for
the Knoevenagel condensation and C–H bond activation reaction
under mild conditions. Using catalysts 1–4 in
Knoevenagel condensation and in C–H activation reaction leads
to the product yields in the range of 95–98 and 97–99%,
respectively. Notably, catalytic performance of 1–4 was also reconnoitered through reusability and percolating experiments
and observed that 1–3 can be reused up to seven
cycles with almost consistent catalytic efficiency, whereas 4 retains its catalytic efficacy up to five cycles. The relative
catalytic efficiency of 1–4 toward the Knoevenagel
condensation reaction has been observed in the order 2 > 1 > 3 > 4, whereas
the
efficiency for C–H activation follows the order 2 > 4 > 1 > 3. Overall,
the
present result demonstrates synthetic, structural, morphological,
optical, and catalytic aspects of CPs 1–4.
Authors: Omar M Yaghi; Michael O'Keeffe; Nathan W Ockwig; Hee K Chae; Mohamed Eddaoudi; Jaheon Kim Journal: Nature Date: 2003-06-12 Impact factor: 49.962
Scheme 1
Chart 1
Scheme 2
Figure 1
Scheme 3
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Scheme 4
Figure 8
Scheme 5
Scheme 6